1. Field of the Invention
Embodiments of the invention relate to an active matrix organic light emitting display, and more particularly to an organic light emitting display and a method of compensating for image quality thereof.
2. Discussion of the Related Art
An active matrix organic light emitting display includes organic light emitting diodes (“OLEDs”) capable of emitting light by itself and has advantages of a fast response time, a high light emitting efficiency, a high luminance, a wide viewing angle, and the like.
The OLED serving as a self-emitting element includes an anode electrode, a cathode electrode, and an organic compound layer formed between the anode electrode and the cathode electrode. The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, a light emitting layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the light emitting layer EML and form excitons. As a result, the light emitting layer EML generates visible light.
The organic light emitting display arranges pixels each including the OLED in a matrix form and adjusts a luminance of the pixels depending on a gray scale of video data. Each pixel includes a driving thin film transistor (TFT) for controlling a driving current flowing in the OLED. It is preferable that electrical characteristics (including a threshold voltage, a mobility, etc.) of the driving TFT are equally designed in all of the pixels. However, in practice, the electrical characteristics of the driving TFTs of the pixels are not uniform by process conditions, a driving environment, and the like. The driving currents from the same data voltage in the pixels are different because of these reasons, and thus a luminance deviation between the pixels is generated. A compensation technology of the image quality has been known so as to solve the problem. The compensation technology senses a characteristic parameter (for example, the threshold voltage, the mobility, etc.) of the driving TFT of each pixel and properly corrects input data based on the sensing result, thereby reducing the non-uniformity of the luminances.
In the related art image quality compensation technology, a method for sensing a change amount of the threshold voltage of the driving TFT and a sensing period thereof are different from a method for sensing a change amount of the mobility of the driving TFT and a sensing period thereof.
As shown in FIGS. 1 and 2A, a sensing method 1 for extracting a change in a threshold voltage Vth of a driving TFT DT detects a source voltage Vs of the driving TFT DT as a sensing voltage VsenA after operating the driving TFT DT in a source follower manner, and detects a change amount of the threshold voltage Vth of the driving TFT DT based on the sensing voltage VsenA. The change amount of the threshold voltage Vth of the driving TFT DT is determined depending on a magnitude of the sensing voltage VsenA, and an offset value for data compensation is obtained through this. In the sensing method 1, after a gate-source voltage Vgs of the driving TFT DT operating in the source follower manner reaches a saturation state (where a drain-source current of the driving TFT DT becomes zero), a sensing operation has to be performed. Therefore, the sensing method 1 is characterized in that time required in the sensing operation is long, and a sensing speed is slow. The sensing method 1 is called a slow mode sensing method.
As shown in FIGS. 1 and 2B, a sensing method 2 for extracting a change in a mobility μ of the driving TFT DT applies a predetermined voltage Vdata+X (where X is a voltage according to the compensation of the offset value) greater than the threshold voltage Vth of the driving TFT DT to a gate electrode of the driving TFT DT, so as to prescribe characteristic of a current capability except the threshold voltage Vth of the driving TFT DT. Hence, the driving TFT DT is turned on. In this state, the sensing method 2 detects the source voltage Vs of the driving TFT DT, which is charged for a predetermined period of time, as a sensing voltage VsenB. The change amount of the mobility μ of the driving TFT DT is determined depending on a magnitude of the sensing voltage VsenB, and a gain value for data compensation is obtained through this. Because the sensing method 2 is performed in the turned-on state of the driving TFT DT, the sensing method 2 is characterized in that time required in the sensing operation is short, and a sensing speed is fast. The sensing method 2 is called a fast mode sensing method.
Because the sensing speed in the slow mode sensing method is slow, a sufficient sensing period is required. Namely, the slow mode sensing method for sensing the threshold voltage Vth of the driving TFT DT may be performed only during a first sensing period, which ranges from after an end of an image display to before the turn-off of a driving power in response to a power-off instruction signal received from a user, so that a sufficient sensing time can be assigned to the sensing operation without the recognition of the user. On the other hand, because the sensing speed in the fast mode sensing method for sensing the mobility μ of the driving TFT DT is fast, the fast mode sensing method may be performed during a second sensing period, which ranges from after the turn-on of the driving power to before the image display in response to a power-on instruction signal received from the user, or during vertical blank periods belonging to an image display driving period.
The offset value updated during the first sensing period and the gain value updated during the second sensing period affect each other. Namely, the gain value is obtained based on a data voltage, in which the offset value is reflected. Thus, the offset value updated in a power-off process has to be stored in a nonvolatile memory, so that the updated offset value can be used when the gain value is determined after a subsequent power-on process. As described above, in the related art compensation technology of image quality, the different sensing methods have to be used to find out the change amount of the threshold voltage and the change amount of the mobility. Therefore, the long time is required in the sensing operation, and the separate nonvolatile memory for storing the offset value is additionally needed and results in an increase in an amount of memory used.
Because the long time is required to sense the change amount of the threshold voltage, it is impossible to sense the change amount of the threshold voltage in a vertical blank period, which is disposed between adjacent image frames and has a relatively short length and in which an image is not displayed. Thus, when the organic light emitting display is driven for a long time and continuously displays an image, the related art image quality compensation technology cannot update the offset value based on the change amount of the threshold voltage. As a result, it is impossible to properly compensate for the change characteristic of the threshold voltage over a driving time.
FIG. 3 shows a change in the threshold voltage Vth of the driving TFT as well as a change in the mobility μ of the driving TFT over a driving time. When a temperature of the display panel rises because of the long time drive of the organic light emitting display, both the threshold voltage Vth and the mobility μ of the real driving TFT change. It is a matter of course that the change amount of the threshold voltage Vth of the driving TFT is less than the change amount of the mobility μ of the driving TFT depending on a rise in the temperature. However, even if the change amount of the threshold voltage Vth is small at a low gray level as compared with a high gray level, the change amount of the threshold voltage Vth may have a relatively large influence on a change in a pixel current. Therefore, the change amount of the threshold voltage Vth of the driving TFT is important. As can be shown from FIG. 3, a change ratio of the pixel current largely depends on the change amount of the threshold voltage Vth at the low gray level. For example, the change ratio of the pixel current depending on the change amount of the threshold voltage Vth was about 155% at the low gray level ‘31’ and was greater than the change ratio ‘137%’ of the pixel current depending on the change amount of the mobility μ at the low gray level ‘31’. When the change in the threshold voltage Vth is not properly compensated, the non-uniformity of the pixel currents is generated. Therefore, a new compensation measure capable of compensating for the threshold voltage Vth as well as the mobility μ for a short period of time is required.